The source of radiation is either an X-ray tube or a pellet of radioactive material emitting gamma-radiation. X-ray equipment is usually described by the electrical voltage across the X-ray tube: thus, 300 kV X-rays. The higher the voltage, the greater the penetrating power of the radiation; industrial X-ray equipment ranges from about 20 kV to 20 MV and the most powerful equipments can be used to radiograph up to 500 mm (20″) steel.
Nearly all gamma-radiography is done with either cobalt-60 or iridium-192 sources; there are a few other radioactive isotopes suitable for gamma-radiography, for special applications.
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